sstable: simplify key reader
sstable: simplify key reader

Now that #475 is solved an read_indexes() guarantees to return disjont
sets of keys sstable key reader can be simplified, namely, only two key
lookups are needed (the first and the last one) and there is no need for
range splitting.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

raphaelsc/scylla,duarten/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla